Sebastian Tomczak is an Australian musician/programmer/video game hacker. This video shows how he turned a laser pointer, a bowl of water, a solar panel and (I'm guessing) a Max/MSP patch into a (nearly) musical instrument. He's developed the concept into the Toriton Plus, with five lasers. It's beautiful, although I get nervous seeing that bowl of water next to a laptop... (Thanks, Steven)

You could probably use a webcam instead of a solar cell (and do some computer vision stuff with it) to get some more interesting results. Solar cell just gets intensity but a video camera could capture the 2d image..